Influence of Directional Solidification on the Creep Properties of a Binary NbSi Eutectic Alloy

The influence of directional solidification (DS) on the creep properties of the binary NbSi eutectic, Nb-18 at.% Si, consisting of Nb solid solution and the Nb3Si silicide phase, is investigated. It is comparatively assessed with an identical alloy composition produced by means of powder metallurgy (PM). The creep behavior is determined for both materials for temperatures up to 1400A degrees C and stresses up to 150 MPa. It is found that the DS process leads to an outstanding improvement of the creep resistance of about three orders of magnitude in terms of minimum creep rate; this significant difference can mainly be attributed to the different sizes of microstructural features (phase size in PM versus cell size in DS material). Diffusional creep may be the responsible creep mechanism in both the PM and the DS material. The activation energies that are responsible for the temperature dependence of creep are found to be very high compared with literature data. It is believed that the contribution of the two phases leads to this high creep activation energy. Comparison with a state-of-the-art single-crystalline nickel-based superalloy (CMSX 4) underpins the outstanding improvement of the creep resistance of DS NbSi eutectics.